Two speed induction motors are well known. Such motors typically are utilized for applications such as furnace blower motors where, under certain predetermined conditions, a high or low motor speed is required. For example, in some furnace systems, the furnace includes a blower fan which selectively operates in low and high fire modes. The blower fan rotates at a slower speed in the low fire mode and rotates at a higher speed in the high fire mode. A motor which drives the blower fan therefore operates at a low speed for the low fire mode and at a high speed for the high fire mode.
Two speed induction motors which may be used in applications such as furnace blower motors typically include stators having two main windings and one start winding. A first main winding forms a first, lower, number of poles and a second main winding forms a second, higher, number of poles. The start winding forms the first number of poles.
The rotor may be a "squirrel cage" type rotor. Particularly, such a rotor includes a rotor core formed by a plurality of laminations. The rotor shaft axis of course is coaxial with the rotor core axis of rotation. Short circuited secondary conductors extend through the rotor core and are arranged axially with respect to the rotor shaft at the outer periphery of the rotor.
In initial operation, the start winding and the first main winding are energized. The magnetic fields generated by such windings induce currents in the moving secondary conductors of the rotor. As is well known, the magnetic fields generated by such windings and the current carrying secondary conductors couple and create a torque which causes rotation of the rotor.
Once sufficient rotor speed is attained, e.g. the rotor speed exceeds the synchronous speed of the second main winding, the start winding and the first main winding are de-energized and the second main winding is energized, i.e., for low fire mode. Typically, the motor will continue operating with only the second main winding energized, i.e., the low fire mode. Under certain conditions, however, the first main winding is energized and the second main winding is de-energized. For example, if the furnace is required to operate in the high fire mode on a particularly cold day, the furnace blower motor will operate with the first main winding energized and the second main winding is de-energized. With the first main winding energized, the motor speed is increased as compared to the motor speed when the second main winding is energized. When warmer weather returns, the furnace operates in the low fire mode with only the motor second main winding energized.
Although known conventional induction motors are relatively quiet as compared to other types of known motors, the rotor in an induction motor rotates at a speed less than synchronous speed. For example, in the case of a six pole induction motor, the synchronous speed (for sixty hertz operation) is 1200 rpm. The rotor may, however, have an actual speed of 1100 rpm. Such a condition is known as "slip" and results in losses associated with induction type motors. Since these losses occur regardless of the operational speed of the motor, such losses are particularly undesirable if the motor runs for extended periods of time, such as a furnace blower motor.
As compared to two speed induction motors, two speed synchronous motors (e.g., permanent magnet motors) typically are more efficient because slip is eliminated. For example, with known permanent magnet synchronous A.C. motors, the stator includes a start winding and a main winding. Permanent magnets are secured at the outer circumference of the rotor. The permanent magnets are magnetized to form a number of poles equal to the number of poles formed by the main winding.
In initial operation, the magnetic fields generated by the start and main windings and the magnetic fields of the rotor permanent magnets couple and produce the torque necessary to cause rotor rotation. Once sufficient rotor speed is achieved, the start winding is de-energized. The rotor continues to rotate due to the coupling between the magnetic fields of the main winding and permanent magnets. No energy is lost as a result of having to induce currents in secondary conductors.
With permanent magnet motors, and in order to operate such motors at two speeds, a frequency controller is required. Specifically, the rotor permanent magnets form a fixed number a poles. Therefore, since the number of poles is fixed, in order to change the rotor speed, the supply voltage frequency must be changed. Control circuitry for controlling the frequency of the supply voltage can be complex and expensive.
In addition, with permanent magnet synchronous AC motors, at motor start-up, significant parasitic torque is generated. Specifically, at motor start-up, the fields of the stator windings and the permanent magnets attempt to cause the rotor to instantaneously transition from a standstill condition to a condition at which the rotor is rotating at synchronous speed. Of course, the rotor cannot make such an instantaneous transition.
The significant forces acting on the permanent magnet rotor at motor start-up result in the generation of undesired noise and vibration. Such noise and vibration are highly undesirable, particularly in applications such as a furnace blower motor or other air-moving applications where such noise can be disruptive and annoying. The vibration can also reduce the operating life of the motor. Therefore, although permanent magnet synchronous AC motors have improved run efficiency as compared to induction motors, such permanent magnet motors have undesirable start-up characteristics.
Accordingly, it is desirable and advantageous to provide a two speed electric motor which is more efficient than known two speed induction motors. Also, it is desirable and advantageous to provide such a motor which does not require a supply voltage frequency controller to change the motor speed. Further, it is desirable and advantageous to provide an electric motor which can exhibit the run efficiency of a permanent magnet motor and the starting characteristics of an induction motor. Such an electric motor, of course, must also be cost effective to manufacture and use so that manufacturing cost increases associated with manufacturing such a motor may be significantly offset by the savings which result from use of such motor.
An object of the present invention is to provide a two speed electric motor having a squirrel cage rotor and which is more efficient than known two speed induction motors.
Another object of the present invention is to provide such a motor which includes rotor permanent magnets but does not require a supply voltage frequency controller.
Yet another object of the present invention is to provide an electric motor which has the running efficiency advantages of a permanent magnet synchronous AC motor but does not have the adverse starting characteristics of a permanent magnet AC motor.
Still another object of the present invention is to provide such an electric motor which, during at least one mode of running condition, does not have the slip, or losses, associated with typical induction run motors.
Still yet another object of the present invention is to provide such an electric motor which is cost effective to manufacture and use.